Sarah Tonkin, 37, was on her way home from her aunt's funeral on Friday with her eight-month-old son, Austin, when she was killed on the Geelong-Bacchus Marsh Road at Balliang East.

Crash investigators believe a 10kg part of a tow bar, called a pintle hook, fell from a truck and was flicked into the path of Ms Tonkin's car by another passing truck.

The pintle hook smashed through the new mother's windscreen and killed her instantly, before her Hyundai SUV travelled 80 metres, struck a tree and rolled over.

Baby Austin escaped with a scratch.

Ms Tonkin's father Richard told reporters on Wednesday his daughter was in the "wrong place at the wrong time by about a nanosecond".

He had been with her at the funeral and waited with her as she asked to use the church bathroom before the drive home.

"We must've waited about two minutes for one of the church staff to come around," Mr Tonkin said.

"The what-ifs and why and all that stuff, you ask all those questions. If she hadn't have asked the chap and if she'd gone straight into the bathroom, those vital seconds would have passed and it wouldn't have happened."

Mr Tonkin said the hardest part was knowing Austin will never know his mother.

"They say in a month he'll forget about it," he said.

Detective Acting Sergeant Phil Frith said it was one of the most tragic cases he had seen.

"It is such a rare event," he said.

"You probably have better chances to win TattsLotto five times for this to occur. It is just one-in-a-million chance."

Police have released CCTV vision of two B-double trucks involved in the crash and are keen to speak to the drivers.

"We don't have any reason to believe that the drivers are aware that they've actually been involved in this incident," the detective said.

"We're not looking at this stage to prosecute anyone, but it is important for Richard and his family that we are able to explain how this tragic indecent has occurred."

A GoFundMe page set up to raise money for Ms Tonkin's partner, Gregor, and their son has raised more than $20,000 in less than 24 hours.

-AAP

EARLIER: Detectives have appealed to the public for assistance with their investigation into a fatal car accident last Friday.

Sarah Tonkin, a 37-year-old Jan Juc mother, died while driving along the Geelong-Bacchus Marsh Road at Balliang East on the afternoon of Friday, December 13.

Detectives from the Major Collision Investigation Unit believe a metal towing component, which they believe had fallen from an unknown vehicle travelling north along the road about 2.15pm, was flicked up by a white b-double tip truck also travelling north into the path of Ms Tonkin's grey Hyundai SUV, which was travelling in the opposite direction.

READ MORE: 'WE HAVE BEEN CRYING FOR 24 HOURS'

The object smashed through the windscreen and struck Ms Tonkin.

Her vehicle continued for approximately eighty metres before it ran off the road, struck a tree and rolled.

Ms Tonkin died at the scene and her infant son, Austin, who was a passenger in the vehicle, was taken to hospital with minor injuries.

Detectives are keen to speak to the driver of a vehicle who may have been driving on the Geelong-Bacchus Marsh Road at the time and is missing a towing component as well as the driver of the white b-double tip-truck.

The tragedy comes after another fatal crash in late October, in which a man and a woman from two separate vehicles were killed.

Ms Tonkin's father Richard Tonkin last week told The Age newspaper that his daughter was a "beautiful girl, just one of those people who walked into a room and lit it up".

"If she happened to be on the road a minute before or a minute after, it wouldn't have happened," he said.

"We have been crying for 24 hours since."

Anyone who witnessed the crash, has dash cam footage or any information is urged to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppersvic.com.au

Have you signed up to The Courier's variety of news emails? You can register below and make sure you are up to date with everything that's happening in Ballarat.